practice. But in the interval, a very large amount of valuable work had been done by Hofmann as well as by others which all contributed to the solution of the problem. It should especially be mentioned, that the study of the behaviour of rosaniline with nitrous acid, to which Hofmann first drew attention in 1862, practically gave the clue to its composition.

**

The attempt has been made in the course of the foregoing account to broadly trace the gradual development of Hofmann's researches on amines and ammonium compounds and the allied phosphorus derivatives, the record of which necessarily forms the chief chapter in a history of his scientific labours. At the time of his departure from England, he had practically completed his studies of this class of compounds; indeed, with the notable exception of the research on primary and secondary phosphines, almost all of his contributions to our knowledge of the chemistry of aminoidsincluding those on colouring matters, of which Dr. Perkin has given so interesting an account-were the outcome of work done during a period of less than 20 years in the modest quarters and with the modest appliances afforded by the Royal College of Chemistry in Oxford Street, the exterior of which is shown in the figure opposite.

The building had a frontage of only 34 ft., its depth being 53 ft. The whole of the first floor was occupied by the students' laboratory, the bench at which the seniors worked being in front of the upper row of windows seen in the figure. On the ground floor, the space on the right of the hall passage was devoted to the Professor's use as a private laboratory, his private room being on the left at the back of the building; the room in front was the balance room and library. The basement was used for larger and rough operations, and contained furnaces, a steam boiler, &c. The lecture theatre was built out at the back. The porter kept apparatus for the use of students, chemicals, &c., in the cellars under the street pavement.

But the record of his activity as an English Professor would be altogether incomplete, and the extent of his services to chemical science would not be sufficiently indicated, if some reference were not made to other work emanating from the Oxford Street laboratory.

Hofmann, as Dr. Perkin reminds us, was practically the discoverer of benzene in coal tar. We must go back fully 50 years to find the record of the discovery in a modest note in Liebig's Annalen (September, 1845, 55, 200), "Ueber eine sichere Reaction auf Benzol; von Dr. Aug. Wilh. Hofmann, Privatdocenten der Chemie an der Universität, Bonu."

The reaction described consists in converting the benzene into nitro

benzene, then reducing by means of acid and zinc,* and testing for the aniline with bleaching solution. It is shown that a single drop suffices for the test, and that in this way it is possible to detect the presence of benzene in a variety of products.

Events, as we know, have proved the discovery to be one of enormous practical importance; the following passage, in which it is recorded, is therefore of interest

"Man findet vielfach in Abhandlungen und Lehrbüchern angegeben, dass das Steinkohlentheeröl Benzol enthalte, allein es ist mir keine Untersuchung bekannt geworden welche sich direkt mit dieser Frage beschäftigt hätte. Von meinen Versuchen über die organischen Basen des Steinkohlentheeröls her, besass ich noch eine Quantität der leichten Kohlenwasserstoffe welche bei der Rectification des rohen Destillationsproduktes des Steinkohlentheers zuerst übergegangen waren. In diesem Oele muste das Benzol enthalten seyn, wenn sich bei der Destillation der Steinkohle überhaupt welches bildet. Ich unterwarf diese Flüssigkeit zu dem Ende einer neuen Destillation; sie gerieth bei 105° ins Sieden, der Siedepunkt stieg aber fortwährend noch höher. Ich sammelte was bis 118° überging und rectificirte das Destillat von Neuem; es begann jetzt schon bei 94° zu sieden und der Siedepunkt stieg weit langsamer als zuvor. Was bis zu 105° überdestillirte wurde dem Versuch unterworfen; es liess sich mit Leichtigkeit erkennen, dass diese Flüssigkeit eine grosse Quantität Benzol enthielt."

Hofmann was not the man to leave such an observation unexploited, especially when the necessity of obtaining aniline for his researches pressed upon him, and although he did not personally undertake the investigation, it was in the Oxford Street laboratory that Mansfield, a few years afterwards, carried out his memorable "Researches on Coal Tar" (Chem. Soc. J., 1849, 1, 446), which were attended with so tragic an end-researches which, he tells us, were commenced at the request of Hofmann and conducted under his advice. The investigation appears to have been the first in which the separation of a constituent of a mixture of liquids by fractional distillation was promoted by using a still head kept at a temperature below the boiling point of the less volatile constituents.

Mansfield not only separated benzene from coal tar, but showed how it might be readily prepared on a large scale of any desired degree of purity, of course availing himself of Faraday's previous

*Hofmann appears to have been the first to use a metal together with an acid as a reducing agent, and to recognise the value of "nascent " hydrogen as a means of reducing nitro-compounds. Bechamp's discovery of the use of ferrous salts as reducing agents was not communicated to the French Academy until July 1854, and the use of iron and an acid was described soon afterwards by him in the Annales de Chimie. Such being the case, it is remarkable that the discovery of aromatic diamines was not more easily made by him, and that instead of attemptirg to reduce dinitrobenzene by means of sulphuretted hydrogen only, he did not at once try other reducing agents.

observations. He at the same time pointed out that it might be procured to any extent from coal tar, or from the light naphtha, in which, he said, it had hitherto been " wasting its sweetness on the desert air;" and that the promises which it made of utility were sufficiently numerous to encourage a belief that it might form a special object of manufacture and of commerce.

But the study of his paper promotes mixed feelings. In the introductory portion, in which an account is given of previous investigations of products of destructive distillation, after referring to the dearth of knowledge of coal tar products, he remarks

"It appears somewhat strange that, in this country, where coal tar is so exceedingly plentiful, our chemists should have been contented with the discovery of naphthalene, and should have allowed others less fortunate than ourselves in being able to command abundance of this almost national production, to inform us of the existence at our feet of vast quantities of aniline, of paranaphthalene, and of other remarkable substances; and it appears, perhaps, no less singular that we should have failed as yet in applying them, when discovered, to the practical uses which they will, no doubt, some day claim."

It is in the highest degree remarkable that an aniline colour industry so soon afterwards came into existence and acquired such importance here; that an industry of corresponding magnitude was in consequence founded on Mansfield's researches; that an anthracene industry subsequently arose; and, lastly, that the azo-colour industry, although conceived, we must allow, in the Marburg laboratory, yet passed through its whole period of gestation-first in the College of Chemistry and then in an English brewery-and sprang into being in this country. The record is a glorious one. For a time, at least, we were not open to the reproach conveyed in Mansfield's words, especially during Hofmann's presence among us. But how different is the condition of affairs now. It is fast becoming necessary to go back to the dim records of the past for an account of English victories in such fields; the men who gained them are all but gone, without leaving successors worthy of the name. Hofmann, Griess, Simpson, Maule and Nicholson have no modern English equivalents; but the type has not merely been preserved but multiplied and even improved abroad. Perkin alone we rejoice to retain among us, and still more that he has retired into a higher sphere of usefulness.

Yet coal tar may still be characterised in Mansfield's words as a national production, and it is an appalling fact, that after having successfully established such industries, we should have allowed them largely to slip through our fingers, in consequence of the failure of our industrial chiefs to march with the times. Nor must we forget that Hofman himself warned us at the very outset, when-foreseeing,